1. Field of the Invention
This invention relates to a method for selectively removing selenium ions from an aqueous solution by reducing the ions to elemental selenium. More specifically, the present method utilizes the reducing power of ferrous hydroxide to convert selenium ions to elemental selenium while the ferrous hydroxide is oxidized to ferric hydroxides.
2. Description of the Background
High levels of selenium ions in potable water are detrimental for human health. In geographical areas where selenium ion concentrations are considered a problem, these ions need to be removed from the water sources. A preferred method for attaining this objective is to reduce the ions to elemental selenium. This form of selenium is not soluble in water and may therefore be easily removed.
Up to the present time only limited success has been obtained in removing selenium ions from water systems. Basicaly, known methods may be divided into four categories:
(1) Conventional desalting techniques such as reverse osmosis, ion-exchange and electrodialysis (Maneval, J. E., Kline, G. and Sinkovik, J., "Selenium Removal from Drinking Water by Ion Exchange" EPA-81-02-5401, Municipal Environmental Research Laboratory, Office of Research and Development, Cincinnati, Ohio (1983), Sorg, T. J., Logsdon, G. S., "Treatment Technology to Meet the Interim Primary Drinking Water Regulations for Inorganics: Part 2", J. Amer. Water Works Assoc. (1978)),
(2) Biological Processes (U.S. Pat. No. 4,519,912 to Kauffman et al, Gersberg, R., Brenner, R., and Elkins, B. V., "Removal of Selenium Using Bacteria," California Technology Institute Publication No. CATI/860201 (1986), Doran, J. W., and Alexander, M., "Microbial Transformations of Selenium", Applied and Environmental Microbiology (1977)),
(3) Adsorption techniques such as those using activated alumina or the like (Hingston, F. J., Atkinson, R. J., Possner, A. M. and Quirk, J. P., "Specific Adsorption of Anions", Nature 215:1459-1461 (1967), Hingston, F. J., Possner, A. M., and Quirk, J. P., "Adsorption of Selenite by Geothite", Adv. in Chem. Series 70:82-90 (1986), Ames, L. L., Salter, P. F., McGarrah, J. E. and Walker, B. A., "Selenate-Selenium Sorption on a Columbia River Basalt", Chem. Geol. 43:287 (1979), and
(4) Chemical reduction processes (U.S. Pat. No. 4,405,464 to Baldwin et al (1983), U.S. Pat. No. 3,933,636 to Marchant (1976) and U.S. Pat. No. 3,387,928 to Doumas (1968)).
In general, the greatest disadvantage of the prior art processes known as conventional desalting techniques is the lack of specificity of the processes for selenate-selenium ions with respect to sulfate ions (see Maneval et al, supra). These processes entail a high cost once all the factors are taken into account. In addition, at the end of conventional desalting processes a brine stream rich in toxic selenate ions must be discarded. Although specific for selenate ions, biological processes for the removal of these ions require days rather than minutes for the reduction of selenium ions, achieve only partial reduction and may generate more toxic compounds such as selenomethionine. Indeed, studies reported on LD 50's (dose causing 50% deaths) for striped bass, the amount of selenomethionine is 10,000 more toxic than selenateselenium ions that are being removed. The adsorption processes seem to involve the formation of insoluble complexes such as the ferric-selenite complex (Chau, Y. K. and Riley, J. P., Chin, ACTA 33:36-49 (1965), Decarla, E. H., Zeitlin, H., and Fernado, Q., Anal. Chem. 53:1104-1107 (1981)). Selenite ions form the insoluble complexes but selenate ions do not form such complexes. Adsorption processes do not show any specificity for selenate ions over sulfate ions. Chemical reduction techniques are known which utilize iron and zinc. However, Baldwin et al, supra, clearly specify metallic iron as necessary for the reductant and operate their process under acidic pH conditions instead of the alkaline conditions needed with ferrous hydroxide. Baldwin et al reduces the concentration of selenium ions in the Se VI oxidation state in an aqueous solution. The aqueous solution is admixed with a quantity of metallic iron which reduces the seenium ions to a lower oxidation state. The dissolved metallic iron is then hydrolized to form an iron hydroxide which precipitates and can thus be separated from the solution. On the other hand, zinc is known to be toxic to fish.
Other methods for removing selenium ions from a solution are also known. U.S. Pat. No. 3,084,994 discloses a method for recovering selenium ions from a selenium quartz compound-enriched solution. An aqueous solution containing selenium ions and having a pH of 7-9 is oxidized in the presence of air or oxygen over a copper oxide, iron oxide or nickel oxide catalyst. U.S. Pat. No. 3,959,097 to Queneau discloses a method for removing trace amounts of selenium from a metal sulfide concentrate using a selenium collector such as a source of ferrous ions and air to generate ferric hydroxide. U.S. Pat. No. 4,026,797 to Nikolic relates to a method for removing selenium from copper electrolytes by adding nickel powder, cobalt powder or iron powder. Sodium sulfide can also be used along with a subsequent addition of ferric sulfide.
However, the above methods for removing selenium from aqueous solutions have limited applications.
Therefore, a need still exists for an improved method for removing selenium ions from an aqueous solution containing such ions.